You can get the same thing in Java essentially: two classes with the same name but loaded by different class loaders are incompatible. You’d get errors at runtime that you can’t cast X to X. I think they’ve made the exception messages more helpful in more recent Java versions, but in older versions the errors were very confusing to newbies because they didn’t mention the class loader difference which was the actual cause of them

In OCaml I think this could only happen with the interpreter (e.g., an interactive toplevel). At least today the compiler complains if you try to define two types with the same name in the same context.

The problem is that it would happen often in the interpreter when you were interactively trying stuff out, because when you're trying stuff out, you change the definitions of things.

Python actually sort of has the same problem not only when you use an interactive interpreter but even when you reload a module: the new class definitions don't modify the old one, they just get bound to the same name. So it's easy to end up with two alglayout.Vbox classes or two diff.Formula classes in the same interactive interpreter at the same time. But it's much less of a problem in Python because Python usually doesn't care what class things are, just what methods they define, so objects belonging to both classes can coexist peacefully. The usual exception is when you have an isinstance check somewhere.

Yeah if you did this now you’d get something like “this expression is of type t/1 but an expression of type t/2” to disambiguate.

The fact that this problem took 25 years to fix is maybe the more damning thing. It wasn't a bug, exactly; it was a usability problem. Clearer demonstrations of development priorities could hardly be given.

That was mostly a man-power issue. The good news is that nowadays such nonsensical error messages are near the top of my personal development priority for OCaml.

During those 25 years OCaml gained native-code compilers for new architectures, labeled arguments, and polymorphic variants, among other things. How could it be mostly a manpower issue?

Work and time contributed by open source collaborators on their free or academic time cannot be magically converted from one subsystem to another. It is honestly very easy to have progress on the aspects that spark interest while some subsystem are starved from attention when there are no full-time developers working on a project.

That's true, but that doesn't mean there was no manpower to fix that error message; it meant that fixing it wasn't a priority to the people who were working on OCaml at the time. You're just offering an explanation for why it wasn't a priority: fixing it didn't spark their interest.

And the REPL should warn you that if you have a value of type "t/n" it is probably a left-over from a previous definition of type t in the session.